This invention relates to a key telephone apparatus and, more particularly, to an attachment for allowing an intercom station in a key telephone system to obtain access to a trunk telephone line.
Key telephone systems serve to interconnect telephone substations located on a private premises with the telephone lines or trunk lines, as variously termed, supplied to the premises by the local telephone company, as well as to provide an "on-premises" communication system. Typically the key telephone system includes some substations that may access the trunk lines directly and also includes a plurality of substations, which are denoted "intercom substations", for providing communications between locations on the premises. In common key telephone systems, one or more telephone intercom substations are connected via two intercom lines to two common intercom circuits in the key service unit so that any party desiring may pick up the handset at an intercom telephone substation, thereby going "off-hook" and place the substation on the intercom line circuit. In the event that the one intercom circuit is in use, the party would intercept any conversation currently in progress and cannot use that intercom line. The party can then position a selector "push key" provided on the telephone instrument to the position of the second intercom line circuit and access the second intercom line. The user can then use the intercom line to reach another intercom substation. Each intercom substation includes a sounder which may be selectively actuated over a control line either by the operator controlling the key service unit, or by another intercom substation if the party at such substation dials the digits of the intercom substation wherein the key service unit decodes the dialed digit and applies the control signal to the sounder of the called substation.
Customarily the intercom stations do not have access to the trunk telephone lines. Hence while persons having access to the intercom substation are permitted to place a call from one intercom station to another to carry on an on-premises conversation, such persons cannot use the intercom substations for placing calls off the premises. Likewise calls coming in over the trunk lines cannot be connected to the intercom substation. Thus when an incoming outside call is received over an incoming trunk line at the operator's key service unit, the operator ascertains the identity of the person whom the calling party desires to reach. If that person is located at an intercom substation, the operator dials the extension number of that intercom substation, and the key service unit of the system operates its switching devices to select a line relay associated with the units and ten digits of the called intercom substation. When the line relay is actuated an electrical circuit is closed over the control line to the sounder contained in the called intercom substation. The called party in answer picks up the handset and receives the information from the operator and if the called party desires to speak directly to the calling party, the called party must call back over a regular substation. The called party cannot reach the calling party over the intercom line. These key telephone systems are of a conventional and well known structure. Typical of key telephone systems in commercial use are the 1A2 model manufactured by ITT Company, and the Litkey key telephone system distributed by Litton BTS, and the T.I.E. Company key telephone system. Conceivably there are circumstances where it is desirable to allow a telephone intercom substation to directly access a trunk line in order to avoid the aforedescribed inconvenience and to obtain a greater flexibility for an existing key telephone system.